This invention relates to solid state power controllers and, more particularly, to the physical arrangement and method of assembly of controllers in which the main switching function is performed by a solid state switching device mounted in a manner to permit maximum heat transfer in operation and ease of replacement.
Solid state power controllers of the type to which the invention applies are generally characterized by a combination of circuit elements that perform preselected operations between a supply and a load where there may be currents of several amperes or more. The main switching function in most such power controllers, including those sometimes referred to as solid state circuit breakers or solid state relays, are performed by power SCRs although other forms of solid state switching devices may be employed. Among the several requirements of such units are that they be compact because of the necessity to mount them, sometimes in large numbers, in a limited space, such as in aerospace applications. The fact that they carry load current and are in a confined space results in the requirement that special attention be given to heat generated in the devices. This may be difficult because the SCR or other switching device may not conveniently be on a stud or flange mounting, which would provide good heat dissipation, because of insufficient room. Thus, space limitations may dictate the use of SCRs of the isolated case type in which the case of the SCR is electrically at anode potential and must be electrically isolated from its support structure. SCRs of this type are sometimes referred to as the "press in" type.
The power controller unit as a whole is housed in an enclosure of box-like configuration in which one or more encased SCRs are mounted. The box also encloses additional circuit elements that tailor the response of the main switching device according to any electrical parameters (e.g. overcurrent or overvoltage) as required by the system in which it is applied. These additional elements may be mounted on one or more printed circuit boards. The resulting unit may be entirely potted with a material such as an epoxy resin to provide assurance of reliable mechanical support and electrical insulation throughout. Examples of solid state power controllers which may be so assembled are those described in copending Applications Ser. No. 023,361 and Ser. No. 023,368, respectively by W. W. Billings and R. E. Luebrecht and by J. T. Mitchell and W. W. Billings, both filed Mar. 23, 1979 and both assigned to the present assignee now respectively U.S. Pat. Nos. 4,245,184 and 4,245,185, issued Jan. 13, 1981. The present invention will be described generally with reference to the mechanical configuration of such power controllers which may employ various kinds of circuit arrangments which will not be detailed herein.
In prior solid state power controllers of the general type described, it has been a practice to attach the SCRs, of the enclosed case type, to an inner surface of the metal box-like enclosure with an adhesive such as an epoxy resin. This has several drawbacks because heat transfer through the adhesive material is limited to a relatively low amount and such materials are not of high insulation quality. Necessarily, the more adhesive material is applied for satisfactory electrical insulation, the worse the thermal problems become. Because of the high thermal drop across the adhesive, it may be necessary to limit the current employed in units of such construction to a few amperes.
It is also known that the SCRs may be soldered to the housing wall using a metallized disc of beryllia for electrical isolation. This provides good heat transfer but makes it very difficult to remove a faulty SCR after testing because accessibility to the SCR may be too limited.
By way of further background, reference is made to U.S. Pat. No. 4,172,272, Oct. 23, 1979, which has to do with a form of solid state relay package in which the main switching device is carried on a conductive ceramic base that is fixed to the interior of the housing by cementing or soldering.
What is desired is a structural configuration and method of assembly of a solid state power controller that takes into account the multiple aspects of required heat transfer capability, electrical isolation, and ease of assembly and disassembly of the power switching device.
In accordance with the present invention, briefly, a solid state power controller is provided that generally comprises a box-like housing. The SCRs or other power switching devices are not mounted directly to a surface of the housing but rather are mounted on a metallic plate subassembly. This is preferably performed by soldering the case of the SCR to the metal plate using a thermally conductive, electrically insulating layer, such as a beryllia ceramic disc, to provide insulation and excellent heat transfer characteristics. This plate is then held against the inside of the front face of the enclosure. This provides a temporary type of assembly in which the plate subassembly is only lightly held with the front face such as by use of the body of a toggle switch, normally provided on the front face, as a supporting medium. Thus, in this stage the plate can be removed, and SCRs replaced, conveniently and expeditiously.